The present invention relates to magnetic direction sensing systems and particularly those for use in vehicles.
Microprocessor-controlled compass systems utilizing magnetic field sensors are known for implementation in a vehicle. Such systems sense the magnitude of the earth's magnetic field using two channels of measurement. The sensor data, if plotted on an X-Y coordinate plane, would be as shown in FIG. 1. For a properly calibrated compass, the plot of sensor data creates a perfect circle centered around the origin of the coordinate plane when the vehicle travels in a 360.degree. loop, as indicated by Graph A of FIG. 1. The radius of the circle represents the detected earth's magnetic field strength, and the vehicle's compass heading at a particular time during travel is represented by a point on the circle. By calculating the angle which the point forms with the X-Y coordinate plane, the compass heading of the vehicle may be determined.
The earth's magnetic field as sensed by the magnetic field sensors on the vehicle, however, is affected by vehicle magnetism. Vehicular magnetism is partially dependent upon the relative locations and densities of ferrous materials within the vehicle, as well as the presence of electrical accessories that may themselves generate a magnetic field such as motors, or even a roof-mounted magnetic cellular telephone antenna. Further, the manner in which a vehicle responds to changes in magnetic fields also depends upon vehicular magnetism. Vehicular magnetism will cause the magnetic field sensed by the compass channels when the vehicle is heading in a given direction to be either greater or lesser than that expected for a vehicle with no magnetic interference. As a result, the plot of sensor data will be shifted away from the origin of the coordinate plane in some direction, resulting in a pattern such as the circle shown in Graph B of FIG. 1 when the vehicle travels a 360.degree. loop. The magnitude of a shift of sensor data from the origin is proportional to the magnitude of the affect of vehicular magnetism on the readings of the sensors. Although Graph B is shown in FIG. 1 as a circle having the same radius as circle A, it should be noted that vehicular magnetism as well as local environmental magnetism may effect the strength of the magnetic field thereby altering the radius of the circle. Further, the sensed magnetic field strengths that the compass senses in any two directions may be affected to different degrees by such vehicular and environmental magnetism. Thus, Graph B may take the form of an ellipse or other non-circular pattern.
Electronic compass systems may be calibrated to compensate for vehicular magnetism. Such calibration may be performed by detecting the center of an obtained circular plot of data B and subsequently computing the difference in the X and Y coordinates between the center of that circle and the origin of the X-Y axis. The difference in X and Y values may then be used to offset the detected sensor levels prior to computing a heading. Alternatively, a compensation signal may be applied directly to the sensors such that the output of the sensors is that of a properly calibrated system.
Although electronic compass systems are known which automatically and continuously calibrate the compass system after it has been installed in a vehicle, some automatic calibration routines require that the vehicle be driven in at least three 360.degree. loops to ensure accurate compensation. However, because space and time constraints at an assembly plant typically do not permit vehicles to be driven in this many loops, newly-manufactured vehicles having such compass systems, are typically transported to dealerships before the automatic calibration routine is able to properly calibrate the compass. If these vehicles are not subsequently driven in a sufficient number of loops to calibrate the compass prior to delivery to the buyer, the buyer may be led to believe that the compass system is defective. Due to a large number of warranty claims arising under these circumstances, some automobile manufacturers have now required that suppliers of compass systems ensure that they are precalibrated so as to compensate for any expected vehicular magnetism prior to delivery to the dealerships. The precalibration data used to calibrated the compass would, for example, correspond to average compensation data used for vehicles of a particular make and model.
There are, however, problems inherent in shipping precalibrated compasses to assembly plants for subsequent installation in a vehicle. In particular, the compass modules, which may otherwise be identical for each of the various makes and models, must be provided with the appropriate precalibration data for the make and model of the vehicle in which it is to be installed. Thus, the compass modules that are shipped must be separately designated for each make and model in which they are to be installed. This leads to inventory control problems at the assembly plants that manufacture more than one model vehicle. Further, vehicular magnetism can vary considerably from vehicle to vehicle even for vehicles of the same make and model. Such variance in magnetism may arise from various options included in these vehicles, such as larger fuel tanks, engines, or other options which a particular buyer may select. Given this variance in vehicular magnetism, a compass system that is precalibrated correctly for one vehicle may not be properly calibrated for another vehicle of the same model. Thus, absent some mechanism for calibrating such compass systems after installation or tightly controlling each vehicle's magnetic signature, there remains a likelihood that many precalibrated compass systems will not operate properly.
To reduce the extent of variability in vehicular magnetism from one vehicle to another, degaussers are utilized to attempt to provide nearly identical magnetic environments for each vehicle. Such degaussers are, however, expensive and difficult to implement and use effectively. Furthermore, effective degaussing requires the degausser to be brought as close as possible to the vehicle. However, various vehicle options make it very difficult to degauss each vehicle consistently. For example, pick-up trucks may have various different gross vehicle weight ratings. These ratings affect the truck's suspension, and consequently, the truck's build height. Thus, the task of controlling a vehicle's magnetic signature, which was difficult before, now becomes nearly impossible without an expensive moving degausser.
One approach used to attempt to solve some of the above-noted problems is to store precalibration data in the form of a proper pair of sensor readings that the compass sensors should output when the vehicle is headed in a predetermined direction if it were properly calibrated. Then, after the compass system has been installed in the vehicle, the vehicle is oriented in this predetermined direction while a precalibration routine is initiated. The precalibration routine compares the precalibration data to the actual sensor readings and computes the difference, which it subsequently uses as compensation offset data to correct all further actual readings obtained from the compass sensors. For example, the precalibration data may be the desired sensor readings when the vehicle is headed due north. Then, after the compass system has been installed and the vehicle is headed due north, the initiation of the precalibration routine informs the compass system that the vehicle is, in fact, due north such that a microprocessor in the compass system can compute the difference between the actual due north reading of the sensors with the desired due north reading to thereby calculate the compensation offset data.
The above approach requires that the compass precalibration data be consistent from part to part when calibrated in an identical field. Further, the magnetic field that is initially sensed during the initialization of the precalibration routine, must be of known and consistent strength. Additionally, the above approach requires that the initiation of the precalibration routine occur within a specific location within the assembly plant. If the location were ever to change or if the magnetic environment surrounding the specified location were to change, the precalibration data stored in the compasses would have to be changed thereby requiring the assembly plant to return to the OEM supplier any compasses not having the updated precalibration data. The above approach also fails to account for any predictable variations in each vehicle's construction that may affect the strength of the earth's magnetic field as sensed by the compass.